Rupture of Wrinkled Steel Pipelines under Monotonic Bending

• Author / Creator

  Mohajerrahbari, Nima

• Buried steel pipelines are widely used in transportation and distribution of oil/gas products over long distance. The changing geo-environmental conditions over the line may subject the pipelines to unforeseen load combinations and prompt the failure and loss of containment of the pipelines that can pose a serious threat to the environment and public safety. Monotonic ground movements commonly impose axial strain and bending curvature on buried pipelines that provoke local buckling of the steel pipes on the compressive side of the wall. Preceding studies have shown that the rupture of compressive wrinkled wall is unlikely to occur under monotonic ground excitations unless due to the formation of rare changing boundary conditions for the deflected segment of the line which is accompanied by a reversal regime of deformations toward the tearing of the pipe wall. However, limited experiments have shown that the monotonically-increasing curvature can trigger the rupture of the tensile wall on the opposite side of the wrinkling. Rupture of steel pipelines is the ultimate failure of the material under the applied stress fields that is a catastrophic consequence of ductile fracture initiation and propagation. The present study concentrates on the rupture of wrinkled buried pipelines on the tensile side of the cross-section due to a monotonic increase of curvature. In order to understand this material behavior, a series of dogbone coupons, notched round bars, and plain strain grooved plate specimens made of API X65 steel grade were designed and tested to study the post-necking plasticity and the ductile fracture of material in steel pipelines. The experimental results indicated the dependence of the fracture strain on the stress triaxiality and Lode angle. Finite element analysis (FEA) of the experiments was then performed to calibrate the ductile fracture toughness in the form of equivalent plastic strain as a function of stress triaxiality and Lode angle. The results of this hybrid experimental-numerical analysis also revealed that the power law could be successfully used to extrapolate the post-necking stress-strain curve of the material through solving an optimization problem. Moreover, the finite element simulations of four pertinent full-scale tests of steel pipes were conducted to examine the material model and simulation techniques. It was demonstrated that the finite element models fairly simulated the post-buckling behavior of the full-scale specimens and closely predicted the rupture of the wall. The verified finite element models were used to study the effect of key factors that were responsible for the rupture of the tensile wall on the opposite side of the wrinkling. The internal pressure, Y/T (yield strength/tensile strength) ratio, and axial tension were found as the dominant design parameters that affected the final failure mode of the wrinkled pipelines and directly increased the chance of post-buckling rupture. In addition, it was explored that the formation of the “pinned-pinned” condition at the boundaries of the deflected line due to immoderate frictional forces was the most critical scenario which dramatically precipitated the rupture of pipe wall by restraining the axial displacement of the line undergoing monotonically-increasing curvature. Lastly, the coupled effect of key design variables was examined through an extensive parametric study. The critical design factor term was introduced to limit the internal pressure as the transitional state from the compressive failure to the post-buckling rupture failure mode due to an increasing curvature. The parametric study showed that the diameter of the pipe had little effect on the ultimate post-buckling failure mode, whereas, the D/t ratio slightly affected the critical design factor. Therefore, a critical design factor envelope was developed as a sigmoid function of the Y/T ratio for the pinned-pinned boundary conditions. This design criterion serves as the minimum rupture limit state for the wrinkled buried pipelines. Finally, the critical design factor was obtained as a surface versus the Y/T ratio and normalized axial tension for the simply-supported boundary conditions to accommodate the impact of the axial tensile load.

• Subjects / Keywords

  • Rupture
  • Monotonic Bending
  • Ductile Fracture
  • Pipe
  • Buried Steel Pipeline
  • FEA
  • Combined Loading
  • Post-Buckling
  • Finite-element-analysis
  • Wrinkling
  • Nonlinear analysis
  • Local Buckling

• Graduation date

  Spring 2017

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R36970C9R

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.